Transparent optical self-healing-ring communication network

ABSTRACT

An optical self-healing-ring communication network is described which include: a first optical communication line, forming a closed optical path; at least two add/drop nodes for optical signals, optically connected along the line; a second optical communication line forming a closed optical path and optically connected to the optical-signal add/drop nodes. Defined in the network are first and second mutually opposite travel directions of the optical signals, with respect to the position of the optical-signal add/drop nodes. At least one of said nodes further comprises selection means, controlled by the optical signals, for the selective dropping of the optical signals from one of the communication lines. At least one of the optical-signal add/drop nodes further include means for the simultaneous input of at least one optical signal in the first direction along the first communication line and in the second direction along the second communication line.

DESCRIPTION

The present invention relates to a transparent communication network foroptical-signal transmission, having a ring-shaped structure, comprisinga double communication line and nodes located along the line for addingand dropping signals related to one or more communication channels, fromand into the ring respectively.

In the network, generally along optical fibres, signals corresponding todifferent channels, each having a different wavelength, travel together,according to the so-called wavelength division multiplexing (or WDM)technique. The components of one signal having a wavelengthcorresponding to one channel are dropped from the network and addedthereinto in an optical form, at the nodes. The network enablestransmission of optical signals without intermediate conversions to theelectric form and it is therefore transparent to the particularstructure of the elementary information to be transmitted (usuallyelectric signals in a digital form).

In optical-signal transmitting networks the amount of the exchangedinformation is very high. For this reason, a failure can have verysevere consequences in that a very high number of subscribers can bedeprived of the information flow.

Typical failures may be breaking of an optical fibre, because said fibrecan be, for example, accidentally cut by operators not aware of thepresence of same, and a cutoff in the operation of the whole node, duefor example to a fire, lack of electric energy or failure of onecomponent.

In order to face such a situation in an automatic manner and withinsufficiently short periods of time so that the information flow is notinterrupted, self-healing-ring networks have been conceived.

In these networks the different nodes are connected with each other bytwo optical-fibre lines, closed upon themselves to form a ring: aprimary ring line (also referred to as external or working ring) and asecondary ring line (also referred to as internal or protection ring).Under normal conditions signals travel in one direction alone on theprimary line and are dropped from and/or added into the different nodesdepending on the wavelength.

In the case of a node failure, or breaking of a primary-line opticalfibre between an upstream and a downstream node, continuity is restoredby diverting the signal flow from the primary to the secondary line inthe node which is upstream of the breaking (with respect to the signaltravel direction in the primary line) and from the secondary to theprimary line in the downstream node. In the secondary line, signalstravel in the opposite direction with respect to the primary line.

In order to perform these diversions in an automatic manner from onering line to another, two so-called 2×2 directional switches areprovided for use in the nodes, which directional switches are opticalfour-way components having two inputs and two outputs. In a normalconfiguration, the first output is optically connected to the firstinput and the second output is optically connected to the second input,whereas in a switched-over configuration the first output is opticallyconnected to the second input and the second output is opticallyconnected to the first input.

A node of this type is described for example in an article by S. Merli,A. Mariconda and R. de Sanctis entitled "Analisi e dimensionamento di unanello ottico trasparente per sistemi D-WDM, con funzioni diriconfigurazione automatica in caso di rottura dell'anello e diDrop-Insert locale dei canali" (Analysis and dimensioning of atransparent optical ring for D-WDM systems having functions of automaticreconfiguration in case of ring breaking and local Drop-Insert of thechannels), Atti del Convegno FOTONICA '95 (FOTONICA '95 MeetingRecords), Sorrento, IT, May 1995. It enables signals to be diverted tothe secondary ring line in case of breaking of the primary ring line. Inaddition, it enables the optical user unit of the node (typically awavelength-selective optical switch, for adding and dropping the signalsof a channel having a predetermined wavelength) to be bypassed in caseof failure of same, while saving transmission between the remainingnetwork nodes.

Networks of this type require a central unit capable of recognizing andlocalizing an occurred failure (for example, through signalling of thelack of signal by the node located downstream of the failure, which cancarry out switching of the optical signal to the service line in aself-governing manner) and of sending to the node upstream of thefailure, the switching-over command so that it can receive signals fromthe service line. For this reason it is necessary for the central unitto communicate with each of the nodes, also and above all in case offailure in the communication line. Therefore emergency lines forconnection between the nodes and the central unit need to be arranged,which lines must be independent of the ring line for communicationbetween the nodes. These emergency lines (that can be optical,electrical, or radio link lines or lines of any other type) togetherwith the central unit involve an important complication in thecommunication network.

In addition, in network architectures of the above stated type, the timenecessary for transmitting the information about the occurred failure tothe central unit, processing the information by said unit andtransmitting the switching over command to the node upstream of thefailure is to be added to the intervention time by the directionalswitches, establishing the overall time for recovery of the networkfunctionality after a break. This overall recovery time can be muchhigher than the intervention time of a directional switch, presentlyreaching values the order of some milliseconds in switches of theacousto-optical or magneto-optical type or some tens of milliseconds inswitches of the mechanical type.

An optical self-healing-ring network is also described in an article "Auni-directional self-healing ring using WDM technique", by E. Almstromet al., ECOC '94 Conference Proceedings, Florence, IT, 25-29 September,1994, vol. 2, pages 873-875. The optical network contemplates the use inthe nodes of optical switches having more than one input and more thanone output. The network is shaped so that the nodes can close the ringon the protection fibre, as soon as they detect a break along at leastone of the fibres.

According to one aspect, the present invention relates to an opticalself-healing-ring communication network comprising:

one optical communication line forming a first closed optical path;

at least two optical-signal add/drop nodes optically connected alongsaid optical communication line;

a second optical communication line forming a second closed optical pathand optically connected to said optical-signal add/drop nodes;

in which at least one of said nodes comprises controlled selection meansfor selectively dropping said optical signals fron one of said first andsecond communication lines,

wherein at least one of said optical-signal add/drop nodes furthercomprises means for the simultaneous input of at least one opticalsignal into said first communication line and into said secondcommunication line.

In particular, defined in the optical ring communication network is afirst and a second travel direction of said optical signals relative tothe position of said optical-signal add/drop nodes, said first andsecond directions being opposite to each other and said networkcomprises means for the simultaneous input of at least one opticalsignal in said first direction along said first communication line andin said second direction along said second communication line.

Preferably the optical ring communication network comprises one pair ofoptical-signal add/drop nodes, optically connected with each other, inwhich the signals transmitted between the nodes of said first pair havea first wavelength, and at least one of said first and second opticalpaths comprises a second pair of optical-signal add/drop nodes,optically connected with each other, the signals transmitted between thenodes of said second pair having a second wavelength different from saidfirst wavelength.

In particular, in the optical self-healing-ring communication networkaccording to the present invention at least one of said optical-signalnodes comprises:

one optical-signal add/drop unit, connected in series by respective lineinput and output ports to said first optical path, further havingrespective signal add and drop ports;

a second optical-signal add/drop unit, connected in series by respectiveline input and output ports to said second optical path, further havingrespective signal add and drop ports;

a controllable optical switch having a first and a second selectableinputs connected to the signal drop ports of said first and secondoptical-signal add/drop units respectively, and one output;

a terminal line unit having an optical input connected to said output ofsaid optical switch and having two optical outputs connected to thesignal add ports of said first and second optical-signal add/drop units;

means for detecting the presence of the optical signal at said dropports of said first and second optical-signal add/drop units,operatively connected to said controllable optical switch for selectionof a respective one of said first and second inputs.

Preferably, said first and second optical-signal add/drop units eachcomprise a demultiplexing unit of said received optical signals at therespective wavelengths and an optical-signal multiplexing unit at therespective wavelengths, and the outputs of said demultiplexing unitscorresponding to the wavelengths included within said bypass band areselectively connected to corresponding inputs of said multiplexingunits.

In a second aspect, the optical self-healing ring communication networkaccording to the present invention comprises a first and a secondoptical self-healing-ring telecommunication network, as previouslydefined, wherein at least one optical node of said first network isoptically connected to at least one optical node of said second network.

In another aspect, the present invention relates to an opticalself-healing-ring telecommunication method in an opticaltelecommunication network, which comprises feeding at least one opticalsignal from a first add/drop node in a first closed optical pathincluded within said network, to a second add/drop node seriallyconnected in said first closed optical path, characterized in that itfurther comprises the steps of:

simultaneously feeding said optical signal from said first add/drop nodeto said first closed optical path and to a second closed optical pathincluded in said network, said second optical path comprising saidsecond add/drop node serially connected thereinto, and

selectively receiving said at least one optical signal in said secondadd/drop node from one of said first and second closed optical paths.

In particular, said optical telecommunication method is characterized inthat said step of feeding an optical signal to said first and secondclosed optical paths comprises feeding said signal in two oppositedirections with respect to said first add/drop node.

In particular, said step of selectively receiving said optical signal insaid second add/drop node comprises:

detecting the presence of said signal in said first closed optical pathin said second node, and

operatively switching over reception from said first closed optical pathto said second closed optical path in the absence of signal in saidfirst closed optical path.

In a preferred embodiment thereof, the optical telecommunication methodaccording to the present invention is characterized in that said firstand second optical-signal add/drop nodes form a first pair of nodesoptically connected with each other, wherein the signals transmittedbetween the nodes of said first pair have a first wavelength, and atleast one of said first and second optical paths comprises a second pairof optical-signal add/drop nodes, optically connected with each other,the signals transmitted between the nodes of said second pair having asecond wavelength different from said first wavelength.

In particular the optical telecommunication method comprises adding anddropping optical signals having a wavelength included within acommunication band in a first and a second add/drop units, wherein saidfirst and second units comprise optical-signal drop outputs andoptical-signal add inputs and said step of adding and dropping opticalsignals comprises:

receiving optical signals from said first and second closed opticalpaths, respectively;

sending to said drop outputs, the received optical signals of awavelength extending within a predetermined dropping band included inthe communication band;

sending to said first and second closed optical paths respectively, thereceived optical signals of a wavelength extending within apredetermined bypass band, included within the communication band andhaving no overlap with said dropping band;

sending to said first and second closed optical paths respectively, theoptical signals present at said add inputs, of a wavelength includedwithin said dropping band.

More details will become more apparent from the following description,with reference to the accompanying drawings, in which:

FIG. 1 is a diagram of an optical communication network according to thepresent invention;

FIG. 2 is a diagram of an optical node according to the presentinvention for adding/dropping optical signals along an opticalcommunication network;

FIG. 3 is a diagram of a directional coupler;

FIG. 4 is a graph of the passband of a directional coupler;

FIG. 5 is a diagram of an optical node according to the presentinvention for adding/dropping signals at two different wavelengths alongan optical communication network; and

FIG. 6 is a diagram of a reconfigurable node according to the presentinvention for adding/dropping signals at different wavelengths along anoptical communication network.

Diagrammatically shown in FIG. 1 is an optical communication networkaccording to the present invention, adapted to transmit optical signalsat a wavelength included within a predetermined band called acommunication band. Respective nodes for adding and dropping opticalsignals have been denoted by 1, 2, 3 and 4 in said figure.

Denoted by 5, 6, 7, 8 are portions of a main transmission line, adaptedto transmit optical signals from nodes 1, 2, 3, 4 to nodes 2, 3, 4, 1respectively, in the direction shown by arrow 13, called the maindirection, and altogether forming a closed transmission ring named mainring.

Denoted by 9, 10, 11, 12 are portions of a secondary transmission lineadapted to transmit optical signals from nodes 2, 3, 4, 1 to nodes 1, 2,3, 4 respectively, in the direction shown by the arrow 14, called thesecondary direction, and altogether forming a closed transmission ringnamed secondary ring.

The optical-signal add/drop nodes will be described in detail in thefollowing. The transmission line portions are preferably made ofstretches of single-mode optical fibre. The optical fibre pairs (of themain and secondary rings, respectively) connecting the same opticalnodes can be housed in one and the same cable.

While in the following of the present description reference is made tothe case, shown in the figure, of four optical-signal add/drop nodes,the present invention is not limited to this particular number and isintended as extended to different numbers of nodes. In particular, anoptical communication network according to the present invention maycomprise a number of nodes correlated with the number of wavelengthsused for signal transmission. Preferably a wavelength connects a singlepair of optical nodes where signal add/drop at that wavelength iscarried out.

Diagrammatically shown in FIG. 2 is an optical node for optical signalsadd/drop into/from a communication network, adapted for use in thecommunication network described with reference to FIG. 1, for example.

The node uses two optical-signal add/drop units, to/from one portion ofmain transmission line and to/from one portion of secondary transmissionline, respectively. The two units, also referred to as OADM (OpticalAdd-Drop Multiplexer), are identified by 21 and 22 respectively in FIG.2.

The OADM 21 receives at one input thereof 23, the optical signals from aportion of main transmission line, circulating in the main directionalong the main ring.

The OADM 22 receives at one input 24 thereof, the optical signals fromone portion of the secondary transmission line, circulating in thesecondary direction along the secondary ring.

OADMs 21 and 22 are optical devices adapted to separate the incomingoptical signals, based on the respective wavelengths, so as to send tooutputs 25 and 26 respectively, called drop outputs, the signals of awavelength extended within a predetermined band (included within thecommunication band) which is different for each of the nodes and iscalled dropping band, and to send to outputs 27 and 28 respectively,named bypass outputs, the optical signals of a wavelength extendedwithin a predetermined band (included within the communication band)which is different for each of the nodes and has no overlap with thedropping band, named bypass band. OADMs 21 and 22 are also adapted tosend optical signals of a wavelength included within the dropping bandand present at inputs 29 and 30 respectively, named add inputs, to thebypass outputs 27, 28.

In the following description reference will be made to the optical node1 of the communication network shown in FIG. 1, the description beingthe same for the remaining optical nodes.

The communication network can contemplate the transmission betweenoptical nodes of telemetry and service channels. For this purpose it ispossible to use optical signals of a wavelength in a band different fromthe communication band. For example, when the communication bandcorresponds to the third optical telecommunication window, around thewavelength of 1550 nm, it is possible for the transmission of telemetrychannels to use optical signals of a wavelength included within thesecond optical telecommunication window, around the wavelength of 1300nm.

If telemetry channels are present, optical signals reaching the opticalnode from the main transmission line portion 8 can be fed to an opticalcoupler 31, whereas optical signals reaching the node from the secondarytransmission line portion 9 are fed to an optical coupler 32. Opticalcouplers 31 and 32 are wavelength-selective couplers adapted to separatetelemetry signals, with a predetermined telemetry wavelength external tothe telecommunication band, towards respective outputs 31a, 32a bothconnected to a telemetry receiver 50, and signals with wavelengthswithin the telecommunication band towards the other outputs 31b, 32bconnected to inputs 23 and 24 of OADMs 21 and 22, respectively.

As already said, OADMs 21 and 22 send the signals having wavelengthsincluded in the dropping band towards respective drop outputs 25 and 26.These outputs are optically connected to signal-presence signallingdevices 33 and 34, including photodiodes for example, opticallyconnected to the respective drop outputs 25, 26 through correspondingoptical couplers, and therefrom to inputs 35 and 36 of a 2×1 typeoptical switch 37, provided with two inputs and one output.

In particular, optical switch 37 has one output 38 which is opticallyconnected to either one of inputs 35 and 36 depending on a signal from acontrol unit 51 receiving at its input the signal-presence signal fromsignalling devices 33 and 34.

The control unit 51 comprises processing means, a microprocessor forexample, adapted to generate control signals for the switch 37 inresponse to given conditions. This unit can be of a type known in theart and therefore is not further described.

The output 38 of the optical switch 37 is connected to a line terminal39. The line terminal 39 comprises an interface between the opticalcommunication network and the users connected thereto. In particular itcomprises one or more optical receivers for the communication signalsdropped from the network and one or more transmitters, adapted togenerate optical signals to be added into the network, of wavelengthscorresponding to those of the signals dropped from the network. The lineterminal 39 can be of known type and is not further described.

The optical signals generated from the line terminal 39 are sent,possibly through a variable attenuator 49 (adapted to equalize the powerof the emitted signals of the line terminal with the power of thesignals bypassed by the OADM unit and inputted into the network again)to an optical coupler 40, adapted to equally divide the opticalradiation into two outputs connected to the add inputs 29 and 30 of theOADMs 21 and 22, respectively.

As an alternative solution, in place of the individual line terminal 39and optical coupler 40 connected thereto, a pair of identical opticaltransmitters can be used for carrying out the signal separation into thetwo outputs 29, 30, one of said transmitters being used for generatingsignals to be sent into the main ring and the other for generating thesame signals to be sent into the secondary ring. In this case the use oftransmitters having a lower output power will be possible and thetransmitters will be directly connected to the add inputs 29 and 30 ofthe OADMs 21 and 22, and the optical coupler 40 can be dispensed with.

In the following, the line terminal 39, variable attenuator 49 ifpresent, and optical coupler 40, or the alternative embodimentcomprising two transmitters for the same signals, will be referred to asterminal line unit 39A, for the sake of simplicity.

The bypass outputs 27 and 28 of the OADMs 21 and 22 are connected to theoptical amplifiers 41 and 42, respectively, which optical amplifiers mayterminate at the input of wavelength-selective couplers 43 and 44. Thelatter are adapted to combine the signals of a wavelength internal tothe telecommunication band and coming from the optical amplifiers, withrespective telemetry signals at the telemetry wavelength, coming from atelemetry transmitter 52.

Wavelength-selective coupler 43 is connected to main transmission lineportion 5. Wavelength-selective coupler 44 is connected to secondarytransmission line portion 12.

The communication network described with reference to FIGS. 1 and 2enables communication between pairs of optical nodes, by the WDMtechnique. The optical signals (having one or more wavelengths includedwithin the communication band) produced by one of the nodes, referred toin the following as start nodes, having the configuration shown in FIG.2, are inputted to the network at the node itself, along the main ringin the main direction and along the secondary ring in the secondarydirection. Possible intermediate optical nodes can be arranged in a wayto insure substantial signal transparency, without dropping signals atthe same wavelengths from the ring or inserting them thereinto, andoptionally can compensate, by means of an appropriate opticalamplification, the attenuation undergone by signals due to the effect ofpassive components.

When signals reach a destination node, which also has the configurationshown in FIG. 2, they are dropped from the network by the correspondingOADM units present in the node. Under normal operating conditions of thenetwork, signals from the start node are present at the drop outputs ofboth OADM units present in the node, after travelling over the relatedrings in opposite directions. In this case the control unit present inthe node drives the optical switch in such a manner that the output ofthe OADM relative to the main ring (output 25 in FIG. 2) is connected tothe receiver.

If a failure, or even several simultaneous failures, occur along thenetwork, in the stretch between the start and destination nodes in themain direction, due for example to a broken optical fibre in the mainring or to an intermediate optical node out of use, the control unit 51in the destination node recognizes the absence of optical signalarriving from the OADM relative to the main ring and drives the opticalswitch to reception from the OADM unit relative to the secondary ring,where the signals sent from the start station and which have travelledover the network in the opposite direction, that is along the portionnot concerned with the failure, are present.

Therefore in case of failure, operation of the network can be restoredin a self-governed manner by the node itself, by modifying the switchingstate of the optical switch, without it being necessary to receiveorders from a remote central unit and without it being necessary toarrange a connecting network between said central unit and the nodes.The time necessary for restoring the network operation is substantiallycoincident with the switching-over time of each optical switch, sincethe time necessary for detecting the absence of signal from the mainring is generally negligible as compared with the switching time, andsince no communication between remote units is required.

The wavelengths relative to signals dropped from the destination nodecan be re-used downstream of the node, by inserting new signals at thesame wavelengths in the network at the node itself. These signals tooare inputted simultaneously along the main ring in the main directionand along the secondary ring in the secondary direction. Theintermediate nodes, in the main direction, between the destination nodeand start node, must be arranged so that signals at those wavelengthsare neither dropped from the network nor inputted thereinto. Thus acommunication occurs between the destination node and start node, whichcommunication in a quite symmetric way with what previously wasdescribed, is automatically maintained active even in the case offailure along the network. Hence, there is on the whole a bidirectionalcommunication between the start node and the destination node, a singlewavelength of the network communication band for each bidirectionalchannel being reserved.

In addition it is possible to use wavelengths different from those usedfor the communication between the start and destination nodes to carryout further bidirectional communications between different pairs ofnodes.

The described possible operating configurations are given by way ofexample only, since it is possible in the network to carry out signaltransmission between different nodes, with the only expection that it isnot possible to add into a node on the network, signals of a wavelengthcorresponding to a signal already present on the network at the (mainand secondary) outputs of that node and it is therefore necessary thatthe signals at the wavelength in question are dropped from the networkby the node itself.

The optical switch 37 can be for example, depending on the requiredintervention times, model YS-111 manufactured by FDK, having a maximumswitching time of 1 ms, or model S-12-L-9 manufactured by DiCon, havinga maximum switching time of 20 ms.

Optical amplifiers 41 and 42, adapted to amplify the radiation at thesignal wavelengths and to compensate for the attenuation undergone bysaid signals along the optical ring fibres and in the optical nodes,without intermediate conversions to an electric form, preferably are ofthe type comprising an optical fibre with a fluorescent dopant, forexample of the type described in the patent application EP677902,assigned to the assignee of this application, published on Oct. 18,1995.

Adapted for use in the present invention are for example opticalamplifiers model OLA/E-MW, manufactured by the assignee of thisapplication, operating in the 1534-1560 nm wavelength band, with anoutput power ranging between 12 dBm and 14 dBm in the presence of anoverall power of the incoming signals ranging from -20 dBm and -9 dBm.

The number and features of the optical amplifiers, both along the mainring and along the secondary ring, can be selected, following knowntechniques, depending for example on the lengths of the differentoptical line portions forming the two rings, on the attenuation of thefibre used to make them and the optical components passed through by thesignals at the add/drop nodes, and in such a manner that oscillationsarising due to the radiation circulating on the closed optical paths ofeach of the rings is avoided. Optical filters adapted to stop radiationat wavelengths different from the signal wavelengths can be disposed atthe optical amplifiers for attenuating the circulating spontaneousemission.

The OADM units can be made for example using a pair ofwavelength-selective couplers for each of them. In FIG. 2 the selectingcouplers being part of OADM 21 are denoted by 45 and 46, those beingpart of OADM 22 are denoted by 47 and 48.

By wavelength-selective couplers optical components are meant adapted toconvey optical signals at different wavelengths present on two inputfibres, to a single output fibre and respectively to separate signalssuperposed on a single input fibre, into two optical output fibres,depending on the respective wavelengths. Said selective couplers arerequired to have a passband width adapted to enable a separation of thesignals in the two directions, in the absence of crosstalk.

The selective couplers 45, 46, 47, 48 preferably can be of the typeschematically shown in detail in FIG. 3, with four optical access fibres(input or output ports) identified by 101, 102, 103, 104 respectivelyand containing a selective reflecting component 105 in the central partthereof, which component behaves as a band-pass element in transmissionand as a band-stop element in reflect ion, and is therefore adapted tolet the signals of wavelengths within a predetermined band pass and toreflect the signals of wavelengths external to such a band. An inputsignal at fibre 101 of the selective coupler with wavelength λ_(p)internal to the passband of component 105, for example, is transmittedwithout significant attenuation towards fibre 103 and, likewise, signalsof wavelength λ_(p) are transmitted from fibre 104 to fibre 102 or,symmetrically, from fibre 103 to fibre 101 and from fibre 102 to fibre104. An input signal at fibre 101 of a wavelength λ_(r) external to sucha band, on the contrary, is reflected towards fibre 104 and likewisesignals of a wavelength λ_(r) proceed from fibre 102 towards fibre 103and symmetrically from fibre 104 towards fibre 101 and from fibre 103towards fibre 102.

With reference to FIG. 4, shown in the following as the passband of theselective reflecting component 105 or, in a wider sense, as the passbandof the selective coupler, is the band of wavelengths close to a minimumattenuation wavelength in transmission, to which, in the transmissionthrough the selective reflecting component 105, an attenuation notexceeding 0.5 dB in addition to the minimum attenuation corresponds. Thewidth of this passband is indicated in FIG. 4 as "-0.5 dB BW".

In the same manner, shown in the following as the reflected band of theselective reflecting component 105 or, in a wider sense, as thereflected band of the selective coupler, is the band of wavelengthsclose to a minimum attenuation wavelength in reflection to which, in thereflection by the selective reflecting component 105, an attenuation notexceeding 20 dB in addition to the minimum attenuation corresponds.

A configuration of a OADM unit relative to a add/drop node of a signalhaving a wavelength λ₁ in a communication network provided for signalsat the four wavelengths λ₁, λ₂, λ₃, '₄ will be now described, by way ofexample.

The selective couplers are such selected that wavelength λ₁ is includedin the passband and wavelengths λ₂, λ₃, λ₄ are included in the reflectedband.

While the selective couplers have been described with four accessfibres, those adapted to the above use can have only three accessfibres, the fourth (the one denoted by 104, for example) remainingunused.

With reference to FIG. 2, operation of an OADM unit, such as unit 21herein shown, will be described This unit is comprised of two selectivecouplers 45 and 46 connected such that an access fibre of the firstcoupler is optically connected to an access fibre of the second one.Structure and operation of the corresponding OADM unit 22 are quiteidentical.

Among the signals at wavelengths λ₁, λ₂, λ₃, λ₄ present at the input 23of the selective coupler 45, the λ₁ wavelength signal is transmitted tothe output 25 of the selective coupler itself, which output iscoincident with the output 25 of the OADM 21. Signals at the remainingwavelengths λ₂, λ₃, λ₄, are reflected towards the output 53 of theselective coupler 45 optically connected to the input 54 of theselective coupler 46. The same signals are then reflected towards theoutput 27 of the same coupler which is coincident with the output 27 ofthe OADM 21. A signal of wavelength λ₁, present at the input 29 of theselective coupler 46 (or the OADM 21) is reflected towards the output 27and brought to become superposed with the signals at the otherwavelengths, coming from the communication network.

If two nodes located along the network contain OADM units as the onedescribed, having corresponding selective couplers to the samewavelength, the network according to the present invention enables abidirectional communication between these nodes by means of signals atthat wavelength, which communication is quickly activated again in caseof failure along the network. The other wavelengths are available forother bidirectional connections between different nodes in the network,which are also self-healing in case of failure.

By way of example, an appropriate selective coupler is model BWDM xTF1,commercialized by E-TEK DYNAMICS Inc., 1885 Lundy Ave., San Jose, Calif.(U.S.), the structure of which corresponds to the description made withreference to FIG. 3, with the only variant that only three access fibres101, 102, 103 are present. For the selective coupler of the above model,the passband width, as previously defined, is about 4 nm.

While the above description relates to a preferred embodiment comprisingwavelength-selective couplers operating in reflection, the OADM unitscan be made, within the scope of the present invention, using planaracousto-optical devices, other devices in planar optics or other devicesequivalent thereto, making use of optical filters for example. Thedescribed node structure can be also extended so as to enable add/dropof two (or more) signals at different wavelengths in one and the samenode. For doing so it is sufficient, according to what shown in FIG. 5,to modify the optical-node structure as so far described, by adding one(or more) devices 60', similar to the portion of FIG. 2 enclosed withinbox 60, in which the OADM units are suitably selected depending on thewavelengths to be added/dropped.

Numbers with primes will be used to denote the components of theadditional device 60' corresponding to the respective componentsdenoted, during the description related to FIG. 2, by same numberswithout prime.

Outputs 53, 55 of the selective couplers 45, 47 are optically connectedto inputs 23', 24' of selective couplers 45', 47' and inputs 54, 56 ofselective couplers 46, 48 are optically connected to outputs 27', 28' ofselective couplers 46', 48'. In this device, a signal with wavelength λ₂present at the output 53 of the selective coupler 45 is transmitted bythe selective coupler 45' to the output 25' and then processed in thesame manner as described in the case of the device of FIG. 2. A signalat the same wavelength λ₂, generated by the line terminal 39' andpresent at the input 29' of the selective coupler 46' is in turntransmitted, through the output 27' of same, to the input 54 of theselective coupler 46. Signals at the remaining wavelengths λ₃, λ₄, onthe contrary, follow an optical path comprising the output 53 of theselective coupler 45, the input 23' of the selective coupler 45', theoutput 53' of same, connected to the input 54' of the selective coupler46', and then the output 27' of same and the input 54 of the selectivecoupler 46, in which superposition with the signals at the otherwavelengths occurs. The signal path in the corresponding OADM units 22,22' connected with the secondary ring is wholly symmetric with thedescribed one.

The case of add/drop with respect to a single node, of signals at twodifferent wavelengths can be generalized to a higher number of signalsat different wavelengths, by addition of corresponding devices 60'",60'", etc . . .

Another, configurable, optical node for adding/dropping signals along anoptical network according to the present invention will be now describedwith reference to FIG. 6, showing the portion of an optical nodecorresponding to block 60 in FIG. 2. As to the description of theremaining part of the optical node, reference is made to what already isdescribed in connection with that case.

The shown device, generally indicated by 160, comprises tworeconfigurable OADM units 121 and 122.

Each of these units comprises demultiplexing units, denoted by 145 and147, respectively, adapted to divide the input signals, at the relatedwavelengths, into a number of outputs, equal to the number ofwavelengths used for transmission along the network.

The OADM units 121 and 122 each further comprise multiplexing units 146,148, adapted to combine together in a single output, the signals at thedifferent wavelengths fed to a number of respective inputs, equal to thenumber of wavelengths used for transmission along the network.

The outputs of demultiplexing units 145, 147 corresponding to thewavelengths that are not to be dropped from the optical node areoptically connected to the corresponding inputs of multiplexing units146, 148, for example by means of optical fibres 150.

The remaining outputs of demultiplexing units 145, 147 corresponding tothe signals dropped from the network in the optical node are insteadconnected to respective optical switches 37a, 37b, 37c, each of thembeing connected to a line terminal 39a, 39b, 39c, in turn adapted toemit optical signals at the wavelengths of the dropped signals, inputtedby couplers 40a, 40b, 40c to respective inputs of multiplexing units146, 148.

An optical node of this type is reconfigurable without interruptingoperation of same, by modifying the connection of one or more pairs ofoutputs of the demultiplexing units 145, 147 with the correspondinginputs of the multiplexing units 146, 148.

In this manner it is possible to transform the optical node into anadd/drop unit for a specific wavelength, by connecting a switch 37, aline terminal 39 and a coupler 40 between the corresponding outputs ofthe demultiplexing units 145, 147 and the corresponding inputs of themultiplexing units 146, 148.

Likewise, the optical node can be transformed into a bypass unit for agiven wavelength, by optically connecting the corresponding outputs ofdemultiplexing units 145, 147 and the corresponding inputs ofmultiplexing units 146' 148.

Reconfiguration of a network having optical nodes of this type can becarried out without stopping operation of the nodes themselves.

In addition it is possible to use the optical nodes described withreference to FIGS. 2, 5 and 6 for carrying out add/drop of one or moresignals to and from a second ring communication network according to thepresent invention, independent of a first network, of the type so fardescribed. The second network in this case can have optical nodessimilar to those of the first communication network.

One or more wavelengths are reserved, on each of the two networks, forsignals exchanged between the two networks. One or more optical signalsdropped from a node along the first network are added into the secondnetwork at a node thereof and, through the same optical nodes, signalsat the same wavelengths are dropped from the second optical network andadded into the first optical network.

The signal exchange between the two optical nodes of the two networkscan take place by means of optical connections between the correspondingOADM units. The input and output of the OADM located along the main ringof the first network are connected to the input and output respectivelyof the OADM located along the main ring of the second network, whereasthe output and input of the OADM located along the secondary ring of thefirst network are connected to the input and output respectively of theOADM located along the secondary ring of the second network.

In this manner, at the wavelengths of the signals exchanged between thetwo networks, an optical ring-shaped super-network is formed whichcomprises both the connected networks and a bidirectional communicationcan be established by signals of appropriate wavelengths circulatingalong both networks, between any node of the first network and any nodeof the second network. At the remaining wavelengths that are not incommon between the two networks, the two networks operate as described,independently of each other.

Also the super-network formed by the connection is self-healing healingin case of failure in any of the two connected networks and, in thiscase too, recovery from the failure takes place without a control by acentral unit being necessary.

We claim:
 1. An optical self-healing-ring communication networkcomprising:a first optical communication line forming a first closedoptical path; a second optical communication line forming a secondclosed optical path; and at least two optical signal add/drop nodes,optically connected along said first optical communication line and saidsecond optical communication line, wherein at least one of said add/dropnodes comprises:first line input and output ports serially connected tothe first optical communication line; second line input and output portsserially connected to the second optical communication line;transmitting means for inputting at least one optical signal into saidfirst and second optical communication lines through said first andsecond line output ports, respectively; a controllable switch forselection of either the first or the second line input port andincluding an output; and at least one signal presence detector connectedto at least one of the first and the second optical paths such that afailure in at least one of the first and the second opticalcommunication lines is determined, wherein said at least one signalpresence detector supplies a control signal to the controllable switchto cause selection of either the first or the second line input portbased on the results of the failure determination the controllableswitch; and a terminal line unit comprising an optical input connectedto said output of said controllable switch and first and second opticaloutputs connected to the first and second line output ports of first andsecond optical signal add/drop nodes, respectively, of said at least twooptical signal add/drop nodes.
 2. The optical self-healing-ringcommunication network of claim 1, wherein said at least one signalpresence detector comprises at least one optical detector.
 3. Theoptical self-healing-ring communication network of claim 2, wherein saidcontrollable switch comprises a controllable optical switch.
 4. Theoptical self-healing-ring communication network of claim 2, wherein saidleast one optical detector comprises a photodiode.
 5. The opticalself-healing-ring communication network of claim 1, wherein saidcontrollable switch comprises a controllable optical switch.
 6. Theoptical self-healing-ring communication network of claim 5,wherein atleast one of said optical signal add/drop nodes comprises a firstoptical signal add/drop unit serially connected to said first opticalpath by said first line input and output ports and comprising signal addand drop ports, and a second optical signal add/drop unit seriallyconnected to said second optical path by said second line input andoutput ports and comprising signal add and drop ports, and wherein saidcontrollable optical switch has first and second selectable inputsconnected to the signal drop ports of said first and second opticalsignal add/drop units, respectively, and an output.
 7. The opticalself-healing-ring communication network of claim 1 further comprising:afirst pair of optical signal add/drop nodes, optically connected witheach other, wherein the signals transmitted between the first pair ofoptical signal add/drop nodes have a first wavelength, and wherein atleast one of said first and second optical paths comprises a second pairof optical signal add/drop nodes, optically connected with each other,wherein the signals transmitted between the second pair of opticalsignal add/drop nodes have a second wavelength different from said firstwavelength.
 8. The optical self-healing-ring communication network ofclaim 1, wherein said first and second optical paths have oppositesignal traveling directions.
 9. An optical self-healing-ringcommunication method in an optical communication network, wherein saidoptical communication network comprises a first optical communicationline forming a first closed optical path, a second optical communicationline forming a second closed optical path and at least two opticalsignal add/drop nodes optically connected along said first opticalcommunication line and said second optical communication line, themethod comprising:serially connecting said first optical communicationline to first line input and output ports of at least one of the nodes;serially connecting said second communication line to second line inputand output ports of the at least one node; inputting at least oneoptical signal into said first and second optical communication linesthrough said first and second line output ports, respectively;detecting, at the at least one node, signal presence in at least one ofthe first and second line input ports such that a failure in at leastone of the first and second optical communication lines is determined;and supplying a control signal to a controllable switch which is coupledto the first and second line input ports of the at least one node tocontrol selection of either the first or the second line input portbased on the results of the failure determination; sending to said firstand second closed optical paths, respectively, the optical signalspresent at said add inputs having a wavelength included within saiddropping band; and adding and dropping optical signals having awavelength included within respective communication bands of first andsecond optical signal add/drop units included in at least one of saidadd/drop nodes, wherein said first and second units comprise respectivedrop outputs of optical signals and add inputs of optical signals andwherein said step of adding and dropping optical signalscomprises:receiving optical signals from said first and second closedoptical paths, respectively; sending to said drop outputs the receivedoptical signals of a wavelength included within a predetermined droppingband included in the respective communication band; sending to saidfirst and second closed optical paths, respectively, the receivedoptical signals of a wavelength included within a predetermined bypassband included within the respective communication band and having nooverlap with said dropping band; and sending to said first and secondclosed optical paths, respectively, the optical signals present at saidadd inputs having a wavelength included within said dropping band. 10.An optical self-healing-ring communication method in an opticalcommunication network, wherein said optical communication networkcomprises a first optical communication line forming a first closedoptical path, a second optical communication line forming a secondclosed optical path and at least first and second optical signaladd/drop nodes optically connected along said first opticalcommunication line and said second optical communication line, whereinsaid first and second optical signal add/drop nodes form a first pair ofnodes optically connected with each other, wherein the optical signalstransmitted between the first pair of nodes have a first wavelength,wherein at least one of said first and second optical paths comprises asecond pair of optical signal add/drop nodes, optically connected witheach other, and wherein the optical signals transmitted between thesecond pair of nodes have a second wavelength different from said firstwavelength, the method comprising:serially connecting said first opticalcommunication line to first line input and output ports of at least oneof the nodes; serially connecting said second communication line tosecond line input and output ports of the at least one node; inputtingat least one optical signal into said first and second opticalcommunication lines through said first and second line output ports,respectively; detecting, at the at least one node, signal presence in atleast one of the first and second line input ports such that a failurein at least one of the first and second optical communication lines isdetermined; and supplying a control signal to a controllable switchwhich is coupled to the first and second line input ports of the atleast one node to control selection of either the first or the secondline input port based on the results of the failure determination. 11.The optical communication method of claim 10, wherein the step ofdetecting signal presence comprises optically detecting signal presence.12. The optical communication method of claim 11, wherein the step ofsupplying the control signal comprises supplying the control signal to acontrollable optical switch included in the controllable switch.
 13. Theoptical communication method of claim 10, wherein the step of supplyingthe control signal comprises supplying the control signal to acontrollable optical switch included in the controllable switch.
 14. Theoptical communication method of claim 13, wherein the selection ofeither the first or the second input port is performed at substantiallythe same rate as the switching rate of the controllable optical switch.15. The optical communication method of claim 10, wherein said first andsecond optical paths have opposite signal traveling directions.
 16. Anoptical self-healing-ring communication network comprising:a firstoptical communication line forming a first closed optical path; a secondoptical communication line forming a second closed optical path; and atleast two optical signal add/drop nodes, optically connected along saidfirst and second optical communication lines, and wherein at least oneof said nodes comprises:controlled selection means for selectivelydropping said optical signals from at least one of said first and secondcommunication lines; means for simultaneously inputting at least oneoptical signal into said first and second communication lines; a firstoptical signal add/drop unit serially connected to said first opticalpath by line input and output ports and comprising signal add and dropports; a second optical signal add/drop unit serially connected to saidsecond optical path by line input and output ports and comprising signaladd and drop ports; a controllable optical switch having first andsecond selectable inputs connected to the respective signal drop portsof said first and second optical signal add/drop units, and an output; aterminal line unit comprising an optical input connected to said outputof said controllable optical switch and first and second optical outputsconnected to the respective signal add ports of said first and secondoptical signal add/drop units; and means for detecting theoptical-signal presence at said drop ports of said first and secondoptical signal add/drop units, operatively connected to saidcontrollable optical switch for selection of a respective one of saidfirst and second selectable inputs.
 17. The optical self-healing-ringcommunication network of claim 16, wherein each of said first and secondoptical signal add/drop units comprises an input demultiplexing unit andan output multiplexing unit and has a bypass band, and wherein each ofsaid demultiplexing units includes an output corresponding towavelengths included within the corresponding bypass band andselectively connected to a corresponding input of the correspondingmultiplexing unit.
 18. An optical self-healing-ring communication methodin an optical communication network comprising:feeding at least oneoptical signal from a first optical signal add/drop node in a firstclosed optical path included within said network to a second opticalsignal add/drop node serially connected in said first closed opticalpath; simultaneously feeding said optical signal from said firstadd/drop node to said first closed optical path and to a second closedoptical path included in said network, said second optical pathcomprising said second add/drop node serially connected therein;selectively receiving said at least one optical signal in said secondadd/drop node from one of said first and second closed optical paths;and adding and dropping optical signals having a wavelength includedwithin respective communication bands of first and second optical signaladd/drop units included in at least one of the add/drop nodes, whereinsaid first and second units comprise drop outputs of optical signals andadd inputs of optical signals, and wherein said step of adding anddropping optical signals comprises:receiving optical signals from saidfirst and second closed optical paths, respectively; sending to saiddrop outputs the received optical signals of a wavelength comprisedwithin a predetermined dropping band included in the respectivecommunication band; sending to said first and second closed opticalpaths, respectively, the received optical signals of a wavelengthcomprised within a predetermined bypass band included within therespective communication band and having no overlap with said droppingband; and sending to said first and second closed optical paths,respectively, the optical signals present at said add inputs having awavelength included within said dropping band.
 19. An opticalself-healing-ring communication network comprising:a first opticalcommunication line forming a first closed optical path; a second opticalcommunication line forming a second closed optical path; and at leasttwo optical signal add/drop nodes, optically connected along said firstoptical communication line and said second optical communication line,wherein at least one of said add/drop nodes comprises:first line inputand output ports serially connected to the first optical communicationline; second line input and output ports serially connected to thesecond optical communication line; transmitting means for inputting atleast one optical signal into said first and second opticalcommunication lines through said first and second line output ports,respectively; and at least one signal presence detector including anoutput and connected to at least one of the first and the second opticalpaths such that a failure in at least one of the first and the secondoptical communication lines is determined; and a terminal line unitcomprising an optical input coupled to said output of said signalpresence detector and first and second optical outputs connected to thefirst and second line output ports of first and second optical signaladd/drop nodes, respectively, of said at least two optical signaladd/drop nodes.
 20. The optical self-healing-ring communication networkof claim 19, wherein said at least one signal presence detectorcomprises at least one optical detector.
 21. An opticalself-healing-ring telecommunication method in an optical communicationnetwork, wherein said optical self-healing-ring communication networkcomprises a first optical communication line forming a first closedoptical path, a second optical communication line forming a secondclosed optical path and at least first and second optical signaladd/drop nodes optically connected along said first opticalcommunication line and said second optical communication line, whereinsaid first and second optical signal add/drop nodes form a first pair ofnodes optically connected with each other, wherein the optical signalstransmitted between the first pair of nodes have a first wavelength,wherein at least one of said first and second optical paths comprises asecond pair of optical signal add/drop nodes, optically connected witheach other, and wherein the optical signals transmitted between thesecond pair of nodes have a second wavelength different from said firstwavelength, the method comprising:serially connecting said first opticalcommunication line to first line input and output ports of at least oneof the nodes; serially connecting said second communication line tosecond line input and output ports of the at least one node; inputtingat least one optical signal into said first and second opticalcommunication lines through said first and second line output ports,respectively; and detecting, at the at least one node, signal presencein at least one of the first and second line input ports such that afailure in at least one of the first and second optical communicationlines is determined.
 22. The optical communication method of claim 21,wherein the step of detecting signal presence comprises opticallydetecting signal presence.
 23. An optical self-healing-ringcommunication network comprising:a first optical communication lineforming a first closed optical path; a second optical communication lineforming a second closed optical path; and at least two optical signaladd/drop nodes, optically connected along said first opticalcommunication line and said second optical communication line, whereinat least one of said add/drop nodes comprises:first line input andoutput ports serially connected to the first optical communication line;second line input and output ports serially connected to the secondoptical communication line;transmitting means for inputting at least oneoptical signal into said first and second optical communication linesthrough said first and second line output ports, respectively; acontrollable optical switch for selection of either the first or thesecond line input port and including an output; at least one signalpresence detector connected to at least one of the first and the secondoptical paths such that a failure in at least one of the first and thesecond optical communication lines is determined, wherein said at leastone signal presence detector supplies a control signal to thecontrollable switch to cause selection of either the first or the secondline input port based on the results of the failure determination thecontrollable switch; and a first optical signal add/drop unit seriallyconnected to said first optical path by said first line input and outputports and comprising signal add and drop ports, and a second opticalsignal add/drop unit serially connected to said second optical path bysaid second line input and output ports and comprising signal add anddrop ports, wherein said controllable optical switch has first andsecond selectable inputs connected to the signal drop ports of saidfirst and second optical signal add/drop units, respectively.
 24. Theoptical self-healing-ring communication network of claim 23, whereineach of said first and second optical signal add/drop units comprises aninput demultiplexing unit and an output multiplexing unit and has abypass band, and wherein each of said demultiplexing units includes anoutput corresponding to wavelengths included within the correspondingbypass band and selectively connected to a corresponding input of thecorresponding multiplexing unit.